As a reaction for producing (meth)acrylic acid, there is a catalytic vapor phase oxidation method of propylene, propane or isobutylene. In the method for obtaining (meth)acrylic acid by catalytic vapor phase oxidation of propylene, propane or isobutylene, conditions for oxidation to (meth)acrolein and for oxidation to (meth)acrylic acid in the next step are different. Accordingly, there are a two step oxidation process in which the respective oxidation reactions are carried out by different catalysts or in separate reactors, and a process in which plural types of catalysts are packed in one reactor to carry out the oxidation reactions in the single reactor. In such a step for producing (meth)acrolein by catalytic vapor phase oxidation of propylene, propane or isobutylene by using a molecular oxygen and/or a step for producing (meth)acrylic acid by catalytic vapor phase oxidation of (meth)acrolein by using the molecular oxygen (hereinafter sometimes referred to as “process for producing a (meth)acrylic acid compound”), in order to adjust the catalytic activity and to prevent the formation of detonating gas, gaseous water (steam) is mixed to a gas comprising a combustible gas such as propylene and molecular oxygen to obtain a raw material mixed gas.
FIG. 1 is an example of a flow chart for producing acrylic acid by two step oxidation, wherein propylene with steam and air is oxidized in two steps via a first reactor and a second reactor having a molybdenum type catalyst or the like packed, to form an acrylic acid-containing gas. Such an acrylic acid-containing gas is contacted with water in a condensation column (quench column) to obtain an aqueous acrylic acid solution, which is extracted by adding a suitable extraction solvent, in an extraction column, whereupon the extraction solvent is separated in a solvent separation column. Then, acetic acid is separated in an acetic acid separation column to obtain a crude acrylic acid, and further, from the crude acrylic acid, byproducts are separated in a fractionating column to obtain a purified product of acrylic acid.
Further, in recent years, instead of the above solvent extraction method wherein recovery of acrylic acid from the aqueous acrylic acid solution is carried out by means of an extraction solvent, an azeotropic separation method is also employed wherein distillation is carried out by using water and an azeotropic solvent, so that from the top of an azeotropic separation column, an azeotropic mixture comprising water and the azeotropic solvent, is distilled, and from the bottom, acrylic acid is recovered.
In a case where methacrylic acid is produced by a catalytic vapor phase oxidation method, isobutylene is subjected to vapor phase oxidation. In the case of the two step oxidation method, isobutylene is oxidized to methacrylic acid via methacrolein.
In the process for producing a (meth)acrylic acid compound, the composition of a raw material mixed gas comprising a combustible gas such as propylene, molecular oxygen and gaseous water, may change to decrease the yield of the desired product, or the temperature of the catalyst layers may abruptly increase to form hot spots, thus leading to deterioration in the performance of the catalyst or to a damage of the production apparatus.
JP-A-5-229984 discloses a method for producing acrylic acid by oxidizing acrolein, wherein the raw material acrolein is preheated to increase the selectivity for acrylic acid. Further, JP-A-2000-53610 discloses the ratio in a raw material mixed gas and the raw material mixed gas temperature at the reactor inlet to improve the yield of acrylic acid. However, such methods had a problem of deterioration of the catalytic activity or the useful life of the catalyst.
It is an object of the present invention to provide a process for producing (meth)acrolein or (meth)acrylic acid, which comprises reacting propylene, propane or isobutylene by catalytic vapor phase oxidation in an oxidation reactor, wherein the change of the composition of the raw material mixed gas, the abnormal temperature increase in the oxidation reactor, and the deterioration of the catalytic activity or the useful catalyst life is prevented to produce the (meth)acrylic acid compound constantly and efficiently.